Conventionally, for example, in a technique disclosed in Patent Document 1, cloud image data that is taken so as to partially hide land-surface image data in satellite image data is removed and corrected into the land-surface image data that would emerge in the absence of a cloud. In the conventional technique, a satellite image including a cloud image and a reference image in which the same region as the satellite image is taken without including the cloud image are prepared and, based on a relational expression of a DN (Digital Number) value between the images, a land-surface image that replaces the cloud image is computed from the DN value of the region corresponding to the cloud image in the reference image in consideration of land-cover classification.
Patent Document 2 discloses processing in which pixel density values of the land-surface image and the cloud image, included in the same satellite image, are compared to reduce the pixel density value of the cloud image.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-143054    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-322562 (page No. 2)
However, in the conventional techniques, because a shadow is not considered, accuracy of the land-surface image data that replaces the cloud image data disadvantageously tends to deteriorate.
That is, a generation status of the shadow largely changes depending on a time of the day, a solar position that is changed according to a period of time, and an irregular state of a land surface. For example, in the case of the irregular land surface, the shadow becomes large and small with time, and the position of the shadow is also changed. On the other hand, in the case of the flat land surface, the shadow is not generated in principle irrespective of the solar position. In the conventional techniques, in order to obtain the land-surface image data that replaces the cloud image data, a pixel value at a different time is simply referred to or a pixel value in a different point is referred to. Therefore, there is a risk of deteriorating the accuracy of obtained land-surface image data due to the shadow changed by the time of the day or the difference of the point.